Post-fabrication repair method for thin film imager devices

ABSTRACT

A thin film electronic imager device has a repaired area between an upper conductive layer and an underlying component in the array in which portions of the upper conductive layer and a dielectric layer have been removed such that the upper conductive layer is electrically isolated from the underlying component. The repaired area has a bottom level having a surface comprising material of the underlying component and an intermediate step level having a surface comprising the dielectric layer material that extends around the periphery of the repaired area. The lateral dimension of the intermediate step level is greater than the lateral dimension of the bottom level such that the width of the intermediate level step surface is in the range between about 1 μm and 3 μm. Formation of the structure in the repair area is done by removing material by laser ablation to set back the upper conductive layer from the sidewall of the dielectric material in the region in which the defect was excised.

BACKGROUND OF THE INVENTION

This invention relates generally to thin film electronic imager devicesand more particularly to a low noise solid state radiation imager havingrepairable defects between an upper conductive layer disposed andunderlying components in the imager array.

Solid state radiation imagers typically have a scintillator coupled to aphotosensor imager array such that incident radiation of the type to bedetected is absorbed in the scintillator, resulting in the generation oflight, which in turn is detected by the photosensor array. The imagerarray typically comprises a number of components (such as photodiodesand switching transistors) which are coupled to address lines forconducting electrical signals to and from individual components in animager device. A common electrode is disposed over the photosensor arrayto provide the common contact for each photosensor pixel in the array.The electrical signals generated by the photosensors correspond to thenumber of detected photons passing from the scintillator, and thesignals from the respective photosensors are used to reproduceelectronically an image corresponding to the photons detected by thearray of photosensors.

A defect in the photosensor array can adversely affect overallperformance of the thin film imager device. Common defects includeelectrical shorts between different components in the array, such asbetween the common electrode and underlying components (e.g., addresslines or switching transistors), or between respective fi underlyingcomponents in the array (e.g., between a scan address line and a dataaddress line). The components in the army are commonly formed in aseries of steps in which layers of conductive, semiconductive, andinsulative materials are deposited and these layers are patterned toform the desired components; unwanted conductive paths, or defects, canresult from conductive debris left from the fabrication steps.

Given the expense of fabricating thin film electronic imager devices, itis desirable to have devices that are repairable. In particular, it isdesirable to have a device that is repairable after deposition of thecommon electrode, that is, after the fabrication of the array, at whichtime tests can be run to check the electrical operation of the array.Any repairs made at this point typically require that the commonelectrode must be breached (that is, removed to gain access tounderlying components that require repair). Particularly for medicalimagers, in which noise is a critical factor, it is important that therepaired area not have significant residual electrical leakage thatcauses noise in the imager array. One problem with repairs of low noiseimagers, however, is that repairs that involve breaking through thecommon electrode often result in leakage paths between the commonelectrode and underlying components that increase the noise in thearray. This electrical leakage can result in noise of sufficientmagnitude to degrade the performance of the array to the point where thearray does not meet the specifications for use in a radiation imager.

It is accordingly an object of this invention to provide a method ofrepairing a thin film solid state imager device having an upperconductive layer (common electrode) that provides a low-electricalleakage repair area that does not adversely affect array noisecharacteristics.

Another object of this invention is to provide a repair method andrepair structure that increases yield of fabrication.

A still further object of the present invention to provide anefficacious thin film solid state imager device low-leakage repairstructure.

SUMMARY OF THE INVENTION

A thin film electronic imager device in accordance with this inventioncomprises an upper conductive layer disposed in a first level, anunderlying component disposed in a second level, a dielectric materiallayer disposed between the upper conductive layer and the underlyingcomponent in an electrically-insulated region, and a repaired area inthe electrically insulated region. In the repaired area, portions of theupper conductive layer and the dielectric layer have been removed suchthat the upper conductive layer is electrically isolated from theunderlying component. The repaired area has a bottom level having asurface comprising material of the underlying component and anintermediate step level having a surface comprising the dielectric layermaterial that extends around the periphery of the repaired area. Thelateral dimension of the intermediate step level is greater than thelateral dimension of the bottom level such that the width of theintermediate level step surface is in the range between about 1 μm and 3μm. Typically the upper conductive layer is the common electrode of theimager array and comprises indium tin oxide or the like, and thedielectric material layer comprises polyimide.

A method of repairing a defect in a thin film electronic imager inaccordance with this invention includes the steps of removing a firstportion of the upper conductive layer at a repair site; removing adefect region in at least one underlying component so as to excise thedefect; and removing a second portion of the upper conductive ctivelayer around the periphery of the defect region so as to form anintermediate step at a level in the device below the upper conductivelayer. The upper conductive layer is thus set back from the sidewall inthe region in which the defect was excised, with the width of theintermediate step is between 1 μm and 3 μm. The steps of removing theportions of the upper conductive layer and removing the defect regiontypically include ablating the material to be removed with a laser.Following removal of the defect and formation of the intermediate levelstep, a passivation layer is typically deposited over the repair site.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description in conjunction with the accompanying drawingsin which like characters represent like parts throughout the drawings,and in which:

FIGS. 1(A)-1(D) are cross-sectional views illustrating steps in therepair of a defect in an imager device in accordance with thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

A partially-fabricated thin film electronic imager device 100, such as asolid state x-ray radiation imager, typically comprises a imaging array110 (also referred to as a photosensor array), a pertinent portion ofwhich is illustrated in FIG. 1. Array 110 comprises components (notindividually shown) such as photodiodes, address lines, and switchingdevices (such as thin film transistors) that are arranged to generate anelectrical signal in response to incident optical photons that pass froma scintillator (not shown) that is optically coupled to array 110 at alater stage in the fabrication process.

In array 110, an upper conductive layer 120 (also referred to as commonelectrode 120) is disposed over a dielectric material layer 130 that isin turn disposed to electrically insulate upper conductive layer 120from an underlying component 140. Common electrode 120 provides a commoncontact to a plurality of photodiodes (not shown) or the like and thusis disposed over substantially the entire imaging array, overlyingsubstantially every component in the imaging array. Common electrodecomprises an electrically conductive material that is also lighttransmissive so as to allow optical photons to pass from thescintillator to the photosensors; typically the material is atransparent conducting oxide such as indium-tin oxide, gallium-tinoxide, tin oxide, or the like. Indium tin oxide and tin oxide, forexample, can be deposited in a plasma sputtering process to a thicknessin the range between 600 angstroms and 1400 angstroms to form commonelectrode 120.

Dielectric layer 130 typically comprises an organic dielectric such aspolyimide or the like that is deposited in a spin process to a thicknessin the range between 1 μand 2 μm. Alternatively, dielectric layer 130comprises an inorganic dielectric material such as silicon nitride orsilicon oxide. Dielectric layer 130 extends across array 110 to form aregion in which common electrode 120 is electrically insulated fromunderlying components in the array except at contact points on thephotodiode, at which points dielectric layer material is removed so thatthe conductive material of common electrode 120 is disposed inelectrical contact with a surface of the photodiode.

By way of example and not limitation, underlying component 140illustrates a conductive material in the array, such as address line, atransistor electrode, or the like. As used herein, "underlying" refersto a component that is formed in the fabrication process prior to thedeposition of the common electrode 120, and thus is disposed on adifferent level of the sandwich of materials that are deposited tofabricate the components for the imager device. No restriction on theorientation of the device is to be implied by the terminology relatingto levels of the components. Underlying component 140 typicallycomprises a conductive material such as aluminum, gold, titanium,molybdenum or the like, or a heavily doped semiconductive material (suchas n+ type doped amorphous silicon). Alternatively, component 140 maycomprise a layer of semiconductive material, such as the amorphoussilicon comprising the photodiode body.

FIG. 1(A) illustrates imager device 100 at a stage in the fabricationprocess following deposition of the common electrode and prior to themating of the scintillator to imaging army 110. At this point in thefabrication process final checks are made of imaging array 110 to locateelectrical defects; arrays that are not repairable to maintain asatisfactory performance must be rejected. Electrical defects includeelectrical short circuits between underlying components in the array,such as between address lines, between address lines and switchingtransistors, or between the common electrode and underlying components.Open circuit conditions can also occur which require repair and removalof portions of the earlier-deposited components in the array. Each ofthese repair iterations, however, necessarily incur breaching commonelectrode 120 to remove a shorted portion of the common electrode or togain access to defects in underlying components.

A defect 150 is shown in FIG. 1(A) that comprises a piece of conductivematerial debris that presents a short circuit between common electrode120 and underlying component 140. The illustration of defect 150 is madeby way of example and not limitation, and defect 150 is representativeof any short circuit condition; further, if defect 150 (or any othertype of defect requiring repair) is disposed between other underlyingcomponents, the same repair procedures are used to remove the portion ofcommon electrode 120 overlying the defect and produce a low leakagerepaired structure.

In accordance with the present invention, the short-circuit defect inimager array 110 is repaired by removing a first portion 121 of commonelectrode at a repair site (or defect region) 160 on imager array 110.Typically laser ablation is used to remove common electrode firstportion 121. In particular, an excimer laser operated at low powerprovides a finely-focused beam that is very effective in ablating theconductive material of common electrode 120 without simultaneouslyablating other components in the array or causing such a rapidvaporization of the transparent conducting oxide that results inconductive residue being driven into underlying dielectric layer 130.The low power used to remove common electrode first portion 121 istypically in the range between about 50 micro Joules (μJ) and 60 μJ,which is about 15% of full power in a LCM 308 model excimer laser.Operation of the laser at lower power may necessitate several passesover repair site 160 to completely remove the conductive material ofcommon electrode 120 in the repair area.

After removal of common electrode first portion 121, defect 150 andsurrounding portions of dielectric layer 130 are then removed. Laserablation is typically used to effect this removal, such as with anexcimer laser operated at higher power than that used for removal of thecommon electrode material (e.g., at power levels in the range between250 μJ and 350 μJ). Laser ablation typically allows fine resolution inremoving material from the array (that is, the beam can be focused to anarea of about 3 microns square so that the defect can be excised withminimal removal of material surrounding the defect region). Laserablation also typically results in relatively smooth vertical sidewallsin repair site 160. At the conclusion of this first laser ablation step,the repair region of imager array 110 appears as illustrated in FIG.1(B). For the defect illustrated in the Figures, removal of the defectresults in repair site 160 having a bottom surface 162 comprising theconductive material of underlying component 140.

Next, in accordance with this invention, a second portion 123 of commonelectrode 120 is removed to form an intermediate step level 124 (FIG.1(C)) around the periphery of repair site 160. The surface ofintermediate level step 124 comprises the dielectric material ofdielectric layer 130, that is, the conductive material of commonelectrode 120 is removed to expose the dielectric material. The lateraldimension L₁ of bottom surface 162 is less than the lateral dimension L₂of the intermediate step level such that the width W of step 124 isgreater than 1 μm, and typically is in the range between about 1 μm and3 μm. Laser ablation, using the low power levels noted above, is used toremove common electrode second portion 123 to optimize the ablationwhile reducing the conductive material driven into underlying dielectriclayer 130. The setback of common electrode sidewalls 125 from dielectriclayer sidewalls 164 in repair site 160 serves to reduce possible pathsfor electrical leakage between common electrode 120 and underlyingcomponent 140. For many imager arrays (such as those used in medicalimaging), it is desirable that leakage be less than 100 pA in order tominimize array noise. Data from repaired wafers indicate that forrepairs conducted in the conventional manner, that is, without theset-back of the common electrode material around the repair site, onlyabout one-half of the wafers exhibit leakage of 100 pA or less; forwafers repaired in accordance with this invention, about 95% of thewafers exhibit leakage of 100 pA or less. This reduction of leakage iscritical in high resolution x-ray imagers used in medical applications,and can constitute the difference between acceptance or rejection of arepaired imaging array 110 for use in a radiation imaging device.

Following formation of intermediate level step 124, a passivation layer170 is formed over the repaired area to enhance the long termlow-leakage characteristics of repair site 160. Passivation layer 170comprises a dielectric material such as, silicon nitride, silicon oxide,or the like, which is deposited in a plasma enhanced vapor depositionprocess to a thickness of about 1500 angstroms.

The procedure of this invention thus enables a completed imager array110 to be repaired while maintaining low electrical leakage betweencommon electrode 120 and underlying component 140. The procedure of thisinvention is applicable to any repair procedure requiring breachingcommon electrode 120 to gain access to underlying components (whereversuch components are located in layers of the array), thereby allowingrepair of an imaging array after final assembly, resulting in increasedyields of devices from the fabrication process.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. A method of repairing a defect in a thin filmelectronic imager device between an upper conductive layer and anunderlying electrically conductive component disposed in in the deviceso as to limit electrical leakage between the upper conductive layer andthe underlying electrically conductive component, the method comprisingthe steps of:removing a first portion of said upper conductive layer ata repair site; removing a defect region at said repair site in adielectric layer underlying said upper conductive layer so as to excisesaid defect; removing a second portion of said upper conductive layeraround the periphery of said defect region so as to form an intermediatestep in said dielectric layer between said upper conductive layer andsaid defect region such that said upper conductive layer is set backfrom a sidewall defining said defect region, whereby said upperconductive layer is electrically isolated from said underlyingelectrically conductive component so as to reduce electrical leakagetherebetween; and disposing a passivation layer over said upperconductive layer, said intermediate step in said dielectric layer, andsaid defect region.
 2. The method of claim 1 wherein the step ofremoving a first portion of said upper conductive layer at said repairsite comprises the step of ablating the upper conductive layer disposedabove said defect region with a laser.
 3. The method of claim 2 whereinsaid laser comprises an excimer laser.
 4. The method of claim 3 whereinthe step removing said defect region comprises the step of ablatingmaterial in said underlying component so as to remove said defect. 5.The method of claim 4 wherein the step of removing said defect regionfurther comprises the step of removing non-conductive material disposedin said dielectric layer between said upper conductive layer and saidunderlying electrically conductive component.
 6. The method of claim 5wherein the step of removing a second portion of said upper conductivelayer comprises ablating said second portion with a laser so as to formsaid intermediate step.
 7. The method of claim 5 wherein the width ofsaid intermediate step is in a range between about 1 μm and 3 μm.
 8. Themethod of claim 3 wherein said upper conductive layer comprises amaterial selected from the group consisting of indium-tin oxide,gallium-tin oxide, and tin oxide.
 9. The method of claim 8 wherein thestep of removing said first portion of said upper conductive layercomprises passing said laser over said first portion multiple timesusing a power level less than about 60 μJ.